Methods and apparatus for ofdr interrogator monitoring and optimization

ABSTRACT

Example embodiments add an optical amplifier to an multi-channel, continuously swept OFDR measurement system, adjust amplified swept laser output power between rising and falling laser sweeps, and/or utilize portions of a laser sweep in which OFDR measurements are not typically performed to enhance the integrity of the OFDR measurement system, improve the performance and quality of OFDR measurements, and perform additional measurements and tests.

This application claims the priority and benefit of U.S. ProvisionalPatent Application 62/355,957, filed Jun. 29, 2016, entitled “METHODSAND APPARATUS FOR OFDR INTERROGATOR MONITORING AND OPTIMIZATION,” whichis incorporated herein by reference in its entirety.

TECHNICAL FIELD

The technology described in this application relates to OpticalFrequency Domain Reflectometry (OFDR) measurements used for fiber opticshape sensing and to data processing technology to improve the accuracyand reliability of those OFDR measurements.

INTRODUCTION

Optical strain sensing is a technology useful for measuring physicaldeformation of a waveguide caused by, for example, the change intension, compression, or temperature of an optical fiber. A multi-coreoptical fiber is composed of several independent waveguides embeddedwithin a single fiber. A continuous measure of strain along the lengthof a core can be derived by interpreting the optical response of thecore using swept wavelength inteferometery typically in the form ofOptical Frequency Domain Reflectometry (OFDR) measurements. Withknowledge of the relative positions of the cores along the length of thefiber, these independent strain signals may be combined to gain ameasure of the strain profile applied to the multi-core optical fiber.The strain profile of the fiber refers to the measure of applied bendstrain, twist strain, and/or axial strain along the length of the fiberat a high (e.g., less than 50 micrometers) sample resolution.

Previous patents have described OFDR-based shape sensing with multi-coreoptical fibers (e.g., see U.S. Pat. Nos. 7,781,724 and 8,773,650incorporated by reference). Some applications for OFDR-based shapesensing fiber require a high degree of confidence in terms of theaccuracy and reliability of the shape sensing output. A non-limitingexample application is robotic arms used in surgical or otherenvironments.

In OFDR measurement systems, there are three basic elements: light, amedium (e.g., a fiber waveguide) in which the light traverses, and areceiver that detects light and converts it to an electric signal. Eachof these basic elements contributes to the accuracy of the measurementbeing performed. An example fiber optic shape sensing system is shown inFIG. 1 and includes a tunable laser 1 swept to provide light atdifferent frequencies or wavelengths to an optical network 2 coupled toan optical fiber sensor/device under test (DUT) 3 (also referred toherein as “sensor 3” or “DUT 3”). Each scan by the tunable laser over atuning range of wavelengths or frequencies produces a set of OFDRmeasurement data. The optical network 2 is coupled to detection,acquisition, and control electronics 4 that includes detectors toconvert optical information into electrical signals, analog to digitalconverters to convert analog electrical signals to digital electricalsignals, and a field programmable gate array (FPGA) to process acquireddata and control acquisition. The detection, acquisition, and controlelectronics 4 provide outputs to a processor 5 for further processing,such as computing the shape of the fiber, and ultimately output ofinformation from the fiber sensor/DUT 3. The processor 5 may also oralternatively include an FPGA or GPU.

In the example fiber optic shape sensing system shown in FIG. 1, shapemeasurements depend on several factors including the ability toaccurately detect the light reflected from the medium which is amulticore optical fiber sensor. OFDR-based fiber optic shape sensingdetects point-to-point length changes in each core of the fiber sensor,and the accuracy and noise of the OFDR measurement system rely onrepeatable transmission of light into the fiber sensor and repeatabledetection of the reflected light from measurement to measurement. Thedetection of the reflected light at varying frequencies (in OFDR thelaser is swept or scanned through a measurement range of wavelengths orfrequencies) in the optical frequency domain and the time domain areimportant to achieving accurate fiber optic shape measurements. OFDRscan-to-scan differences or inter-scan differences that are not theresult of physical changes in the sensing fiber can introduce error inthe OFDR measurement data if they are not reduced or corrected. Examplesources of error include laser tuning speed variations, optical outputpower fluctuations, interferometer path length changes, core-to-coredelay shifts, and electrical signal delays in the OFDR channels.

Assuming that relative delays of the detection circuit between the coresare constant, phase changes between cores can be interpreted asresulting from a physical change made to the sensing fiber. But theinventors discovered that these delay relationships change over time,vary with temperature, and/or are affected by an interrogation devicefailure. Delay shifts introduced in data acquisition and processingintroduce error into the OFDR measurement, which is wrongly interpretedas a physical change to the sensing fiber shape. In addition, laserscan-to-laser scan optical power level changes can also lead to OFDRmeasurement error. Such optical power level changes can result inmismatched signal-to-noise ratio (SNR) between rising and falling sweepsof the laser and/or varying SNR over the optical frequency range inwhich the OFDR measurement is performed.

SUMMARY

Example embodiments include an optical measurement system that measuresan optical fiber sensor that includes multiple optical cores. A tunablelaser sweeps over a first measurement range of wavelengths, and anoptical amplifier amplifies the swept laser light. An optical networkprovides the amplified swept laser light to the optical fiber sensor andoutput reflected light from the optical fiber sensor associated witheach of the multiple cores. Detection circuitry detects and converts theoutput reflected light from the optical fiber sensor into correspondingelectrical signals. Data processing circuitry controls a gain of theoptical amplifier to control the power of the swept laser light.

The optical amplifier may be implemented for example with anErbium-Doped Fiber Amplifier (EDFA) connected to an output of a pumplaser source and a portion of laser light from an optical splittercoupled to the tunable laser.

An example optical measurement system is an Optical Frequency DomainReflectometry (OFDR) interrogation system. The optical network includesa laser monitor interferometer coupled to the detection circuitry and ameasurement interferometer coupled to the optical fiber sensor and thedetection circuitry. The tunable laser continuously sweeps over thefirst measurement range of wavelengths so that the detection circuitryacquires OFDR measurement data from the optical fiber sensor duringrising and falling sweeps of the tunable laser.

In example implementations, the data processing circuitry controls again of the optical amplifier to compensate for laser power imbalancesor fluctuation during the rising and falling sweeps of the tunablelaser; corrects the gain of the optical amplifier at multiple differentfrequencies in the first measurement range of frequencies; controls again of the optical amplifier to maintain a substantially constant laserpower level over the first measurement range of frequencies; andcontrols a gain of the optical amplifier to a first gain for the risingsweep of the tunable laser and a second different gain for the fallingsweep of the tunable laser.

Other example embodiments include an Optical Frequency DomainReflectometry (OFDR) interrogation system for measuring an optical fibersensor including multiple optical cores. The tunable laser sweeps over afirst measurement range of wavelengths generating a swept laser outputsignal. A modulator adds a known signal to the swept laser output. Anoptical interferometric network provides the amplified swept laser lightto the optical fiber sensor and output reflected light from the opticalfiber sensor associated with each of the multiple cores corresponding tosensor measurement data. Detection circuitry detects and converts theoutput reflected light from the optical fiber sensor into correspondingelectrical signals. Data processing circuitry processes the sensormeasurement data acquired during sweeps of the tunable laser in thefirst measurement range of wavelengths based on the added known signal.

The data processing circuitry may be configured to determine, based onthe added known signal, errors from delays caused by one or more of theoptical interferometric network, the optical fiber sensor, or thedetection circuitry.

Example implementation features include a laser driver, where themodulator is coupled to the output of the laser driver. The modulatormay include a controller coupled to a digital to analog converter whichdrives a voltage-controlled oscillator and a filter to filter an outputfrom the voltage-controlled oscillator to generate the known signal.Another example modulator includes a numerically-controlled oscillatorto generate a binary signal having a most significant bit used toprovide a clock signal and a filter to filter the clock signal togenerate the known signal. The fiber may have N optical cores, N being apositive integer greater than 3, and the modulator includes anumerically-controlled oscillator to generate N phase signalscorresponding to the N optical cores and N−1 phase difference signals. Aphase error in the sensor measurement data may be based on the N−1 phasedifference signals.

The tunable laser sweep may include a rising sweep where the wavelengthincreases from smallest to largest wavelength in the first measurementrange of wavelengths and a falling sweep where the wavelength decreasesfrom largest to the smallest wavelength in the first measurement rangeof wavelengths. The laser sweep includes a turnaround portiontransitioning between the rising and falling sweeps. The modulator maybe controlled to add the known signal to the swept laser output duringthe turnaround portion. Alternatively, the modulator may be controlledto add the known signal to the swept laser output at wavelengths outsidethe first measurement range of wavelengths.

Still further example embodiments include an OFDR interrogation systemhaving a tunable laser that sweeps over a first measurement range ofwavelengths including a rising sweep of the tunable laser where thewavelength increases from smallest to largest wavelength in the firstmeasurement range of wavelengths and a falling sweep of the tunablelaser where the wavelength decreases from largest to the smallestwavelength in the first measurement range of wavelengths. The lasersweep includes a turnaround portion transitioning between the rising andfalling sweeps. An optical interferometric network provides theamplified swept laser light to the optical fiber sensor and outputsreflected light from the optical fiber sensor associated with each ofthe multiple cores corresponding to sensor measurement data. Detectioncircuitry detects and converts the output reflected light from theoptical fiber sensor into corresponding electrical signals. Dataprocessing circuitry processes the sensor measurement data acquiredduring the rising and falling sweeps of the tunable laser in the firstmeasurement range of wavelengths and performs an additional operationduring the turnaround portion of the laser sweep.

For example, the additional operation may be performing additionalmeasurements at swept laser wavelengths other than those in thepredetermined range. In this situation, a sweep rate of the laser forsome of the other swept laser wavelengths may be slower than a sweeprate of the laser for the sweeps of the tunable laser in the firstmeasurement range of wavelengths.

Other example additional operations include balancing a power level ofthe swept laser light in the rising and falling sweeps, makingscatter-based OFDR measurements for the optical fiber sensor, andperforming in-system checks or adjustments in response to systemdynamics.

If the optical fiber sensor includes fiber Bragg gratings that providelight reflections within the first measurement range of wavelengths,another example additional operation includes making scatter-based OFDRmeasurements separated in wavelength from the grating reflections.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an example OFDR measurement system.

FIG. 2 is a graph showing turn around points in a laser sweep or scan.

FIG. 3 shows an example OFDR measurement system in accordance withexample embodiments.

FIG. 4 shows a more detailed example OFDR measurement system inaccordance with example embodiments.

FIG. 5 is a graph showing example falling v. rising hydrogen cyanide(HCN) gas cell power levels in an OFDR system without correction.

FIGS. 6A and 6B are graphs showing an example laser sweep and pump laserpower, respectively.

FIG. 7 is a graph showing example falling v. rising hydrogen cyanide(HCN) gas cell power levels in an OFDR system with correction.

FIG. 8 is a flowchart diagram illustrating example procedures forimplementing an EDFA in an OFDR measurement system and for adjustingpump laser gain to balance the laser power output for rising and fallinglaser sweeps.

FIG. 9 is a diagram of a laser drive system.

FIG. 10 is a diagram that adds modulation to the laser drive system inFIG. 12.

FIG. 11 shows a first example laser modulator approach.

FIG. 12 shows a second example laser modulator.

FIG. 13 is an apparatus for modulation and measurement in accordancewith example embodiments.

FIG. 14 is an example apparatus for phase computation by mixing with thesame signal used to modulate the laser in accordance with exampleembodiments.

FIG. 15 is a flowchart illustrating example procedures for performingdelay monitoring using laser diode ripple injection.

FIGS. 16A and 16B are graphs showing out of band laser modulation beforeand after linearization.

FIGS. 17A and 17B are graphs showing laser modulation at laser tuningturnarounds.

FIG. 18 is a graph showing extended turnaround points for makingadditional measurements in accordance with example embodiments.

FIG. 19 is a graph showing example out of band slow laser sweep portionsin addition to typical faster speed sweep portions in accordance withexample embodiments.

FIGS. 20A and 20B are graphs showing fast sweep portions and slow out ofband sweep portions.

FIG. 21 is a flowchart illustrating example procedures for utilizinglaser sweep edges and turnarounds to perform additional measurements.

FIG. 22 shows an example OFDR measurement system in accordance withexample embodiments.

FIG. 23 shows an example use of a fiber optic shape sensing system to arobotic surgical arm.

DETAILED DESCRIPTION

The following description sets forth specific details, such asparticular embodiments for purposes of explanation and not limitation.But it will be appreciated by one skilled in the art that otherembodiments may be employed apart from these specific details. In someinstances, detailed descriptions of well-known methods, interfaces,circuits, and devices are omitted so as not to obscure the descriptionwith unnecessary detail. Individual blocks are shown in the figurescorresponding to various nodes. Those skilled in the art will appreciatethat the functions of those blocks may be implemented using individualhardware circuits, using software programs and data in conjunction witha suitably programmed digital microprocessor or general purposecomputer, and/or using applications specific integrated circuitry(ASIC), and/or using one or more digital signal processors (DSPs).Software program instructions and data may be stored on anon-transitory, computer-readable storage medium, and when theinstructions are executed by a computer or other suitable processorcontrol, the computer or processor performs the functions associatedwith those instructions.

Thus, for example, it will be appreciated by those skilled in the artthat diagrams herein can represent conceptual views of illustrativecircuitry or other functional units. Similarly, it will be appreciatedthat any flow charts, state transition diagrams, pseudocode, and thelike represent various processes which may be substantially representedin computer-readable medium and so executed by a computer or processor,whether or not such computer or processor is explicitly shown.

The functions of the various illustrated elements may be providedthrough the use of hardware such as circuit hardware and/or hardwarecapable of executing software in the form of coded instructions storedon computer-readable medium. Thus, such functions and illustratedfunctional blocks are to be understood as being eitherhardware-implemented and/or computer-implemented, and thus,machine-implemented.

In terms of hardware implementation, the functional blocks may includeor encompass, without limitation, a digital signal processor (DSP)hardware, a reduced instruction set processor, hardware (e.g., digitalor analog) circuitry including but not limited to application specificintegrated circuit(s) (ASIC) and/or field programmable gate array(s)(FPGA(s)), and (where appropriate) state machines capable of performingsuch functions.

In terms of computer implementation, a computer is generally understoodto comprise one or more processors or one or more controllers, and theterms computer, processor, and controller may be employedinterchangeably. When provided by a computer, processor, or controller,the functions may be provided by a single dedicated computer orprocessor or controller, by a single shared computer or processor orcontroller, or by a plurality of individual computers or processors orcontrollers, some of which may be shared or distributed. Moreover, theterm “processor” or “controller” also refers to other hardware capableof performing such functions and/or executing software, such as theexample hardware recited above.

The technology described in this application determines and reduces oreliminates sources of error affecting fiber optic measurements.

Increased Gain and Gain Balancing in an OFDR System

As described in the introduction, FIG. 1 is an example OFDR measurementsystem. Unamplified light swept over a range of frequencies/wavelengthby a single tunable laser 1 is guided to an optical network 2, andreflected light from the sensor or DUT 3 interferes with lighttraversing a reference path of the optical network 2. The resultinglight is detected and converted to digital form by detection andacquisition electronics 4 and processed in the processor 5 to provide adesired OFDR measurement, e.g., a shape of the fiber sensor 3. In someinstances, the OFDR measurement is performed in one laser sweepdirection, either increasing or decreasing in optical frequency.Depending on the application requirements such as the update rate, sweeprate and sweep range, it is not always suitable to only perform thedesired measurement in a single sweep direction of the laser. Manyapplications require a laser that is continuously swept withrising/increasing and falling/decreasing sweeping.

An example of a continuous laser sweep is illustrated in FIG. 2 whichdepicts the laser's optical frequencies as a function of time. The slopeof this plot represents the laser's sweep rate. For a Rising Sweep, thelaser is tuned from a lower optical frequency to a higher opticalfrequency. For a Falling Sweep, the laser is tuned from a higher opticalfrequency to a lower optical frequency. In addition to these Rising andFalling Sweeps there are additional parts of the sweep referred to inthis application as laser turnarounds, which include parts of the lasersweep from the completion of one measurement to the start of the nextmeasurement. The turnarounds include a continuation of the current sweepbefore a laser's sweep rate is slowed, eventually changing the sweepdirection, and then accelerating in the opposite direction until thedesired sweep rate is reached.

When the fiber sensor 3 is under tension or compression, the fiber coresexperience some amount of lengthening or shortening. Bend, twist, andoverall tension cause changes in the strain measured in the variousfiber cores. A matrix can be formed to describe the relationship betweenthe bend, twist, and strain on the fiber sensor and the strain on eachcore. For example, if four cores in the fiber sensor are used to measureshape, the relationship between the strain in these four cores and theapplied bend, twist, and strain as a function of length is:

$\begin{matrix}{\begin{bmatrix}{ɛ_{1}(z)} \\{ɛ_{2}(z)} \\{ɛ_{3}(z)} \\{ɛ_{4}(z)}\end{bmatrix} = {\begin{bmatrix}{\alpha \; r_{1}{\sin \left( \theta_{1} \right)}} & {{- \alpha}\; r_{1}{\cos \left( \theta_{1} \right)}} & {\beta \; r_{1}^{2}} & 1 \\{\alpha \; r_{2}{\sin \left( \theta_{2} \right)}} & {{- \alpha}\; r_{2}{\cos \left( \theta_{2} \right)}} & {\beta \; r_{2}^{2}} & 1 \\{\alpha \; r_{3}{\sin \left( \theta_{3} \right)}} & {{- \alpha}\; r_{3}{\cos \left( \theta_{3} \right)}} & {\beta \; r_{3}^{2}} & 1 \\{\alpha \; r_{4}{\sin \left( \theta_{4} \right)}} & {{- \alpha}\; r_{4}{\cos \left( \theta_{4} \right)}} & {\beta \; r_{4}^{2}} & 1\end{bmatrix}\begin{bmatrix}{B_{x}(z)} \\{B_{y}(z)} \\{T(z)} \\{E(z)}\end{bmatrix}}} & (1)\end{matrix}$

Here ε_(i)(z) is the strain measured in core i as a function of distancedown the sensor, z, α is a constant relating strain to bend (“bendgain”), β is a constant relating strain to twist (“twist gain”), r_(i)is the radial location of core i with respect to the center of thefiber, θ_(i) is the angular location of core i relative to a referencecore such as core 2 in FIGS. 1A-1C, B_(x)(z) is the bend in the X-Zplane as a function of distance down the sensor (see FIG. 2), B_(y)(z)is the bend in the Y-Z plane as a function of distance, T(z) is thetwist of the sensor as a function of distance, and E(z) is the axialstrain applied to the sensor as a function of distance.

A measurement of the amplitude and phase of the light reflected alongthe length of the fiber sensor with high resolution and high sensitivitymay be achieved using Optical Frequency Domain Reflectometry (OFDR).

In the case of a multiple channel OFDR interrogation system, eachchannel corresponds to a core in a multi-core fiber sensor or DUT. In amultiple channel OFDR interrogation system, it can be advantageous toadd an amplifier to increase the power to each channel. Exampleembodiments add an erbium-doped fiber amplifier (EDFA) into an OFDRsystem to increase the power of the laser light coupled to the fibersensor. But the EDFA introduces a new variable into the OFDR measurementsystem: amplifier gain. When a pump laser in an EDFA is driven by aconstant current source, the gain of the EDFA can vary depending onvarious factors including the direction in which the swept laser istuned, the instantaneous wavelength of the light being amplified, andthe sweep speed or rate of the laser.

An example embodiment of an added EDFA is shown in FIG. 3. The tunablelaser 1, controlled by the detection, acquisition, and controlelectronics 4, is shown with an example wavelength sweep range of 1520nm-1560 nm. A pump laser 7, controlled by the detection, acquisition,and control electronics 4, provides light at a particular wavelength,e.g., 980 nm, to an erbium-doped fiber amplifier (EDFA) 8. An opticalsplitter 6 splits the light from the tunable laser 1 into two paths: oneto the EDFA 8 and one to the reference path of the Optical Network 2.The amplified light from the EDFA 8 is split into each OFDR measurementchannel and guided to each core of a multi-core sensor fiber comprisingfiber sensor 3.

FIG. 4 shows a detailed example embodiment of an EDFA added to anOFDR-based interrogation system for an example 6-core fiber sensor.Light from a frequency tunable laser 16, controlled in this example bythe processor 22 rather than by the data acquisition electronics 20, issplit with 90/10 coupler between a laser monitor interferometer 10 and ameasurement interferometer 12. In the laser monitor interferometer 10,the light is spilt into three paths using a 3×3 coupler. The first pathgoes to a detector to monitor laser power. The second path passesthrough a hydrogen cyanide (HCN) gas cell to a detector to provide anabsolute wavelength reference. The final path goes through an isolatorand another 3×3 coupler to two Faraday rotator mirrors (FRM) with oneleg having a known delay difference from the other. The return signalsfrom this interferometer form I/Q signals. With a phase offset of 120degrees, the I/Q signals are converted to quadrature signals and used tomeasure the change in optical frequency as the laser sweeps.

The light going into the measurement interferometer 12 is split using a90/10 coupler between a reference branch and measurement branch of theinterferometer 12. The light in the reference branch is split into sixreference signals using cascaded couplers. The light in the measurementbranch passes through an isolator and then through a length oferbium-doped fiber. This fiber is pumped with light from a 980 nm pumplaser 18, controlled in this example by the processor 22 rather than bythe data acquisition electronics 20, that couples in through aWavelength Division Multiplexed (WDM) coupler. This combination oferbium-doped fiber and pump laser 18 amplifies the light in themeasurement branch of the interferometer. The light passes throughanother isolator and then through a polarization controller set to flipthe light between two orthogonal (or nearly orthogonal) polarizationstates on subsequent scans. The light is then split with cascadingcouplers into six measurement channels. The returning light is combinedwith the six reference paths using 2×2 couplers. These combined signalsthen pass through polarization beam splitters (PBSs) to two detectors (Sand P) for each channel (C, I, J, K, U, V) input to the data acquisitioncircuitry 20, forming a polarization diverse detection scheme. Thiscreates an interferometric measurement of the light reflected from up tosix cores of a multicore fiber. The six channels (C, I, J, K, U, V) areconnected to each core of a multicore fiber sensor 24 using a fanoutassembly 14 that couples six single core fibers 28 to six cores in amulticore cable 23 connected by a connector 25 to the multicore fibersensor 24. The controller/data processor 22 controls the tunable laser16, the polarization controller, and the polarization beam splitters,and also drives the pump laser 18. The data processor 22 also acquiresand processes the data from each of the photodiode detectors providedfrom the data acquisition circuitry 20.

The recorded data is the reflected amplitude as a function of opticalfrequency for two polarization states, S and P, for each fiber opticcore measured. The controller/data processor 22 linearizes this recordeddata with respect to optical frequency using the data from the lasermonitor interferometer 10 so that it is represented in equal incrementsof optical frequency. The linearized data is Fourier transformed intothe time domain to represent the amplitude and phase of the reflectedlight as a function of optical delay along each fiber core. The S and Pdata from two sequential orthogonal polarization scans are combined tocompensate for birefringence in the fiber cores and form a scalarmeasure of the amplitude and phase of the reflected light from eachcore. This combined complex signal (amplitude and phase) is comparedwith interferometric data recorded in a reference scan, and theresulting phase difference/change for each core is the measured signalthat is used to compute the current shape of the fiber.

The derivatives of the measured phase changes are proportional to thestrains in each core. The proportionality constant, γ_(i), relating thephase to strain in core i is the strain-optic coefficient for that core.Equation 1 can then be expressed as:

$\begin{matrix}{\begin{bmatrix}{\phi_{1}^{\prime}(z)} \\{\phi_{2}^{\prime}(z)} \\{\phi_{3}^{\prime}(z)} \\{\phi_{4}^{\prime}(z)}\end{bmatrix} = {\begin{bmatrix}{\alpha \; {\gamma \;}_{1}r_{1}{\sin \left( \theta_{1} \right)}} & {{- \alpha}\; \gamma_{1}r_{1}{\cos \left( \theta_{1} \right)}} & {\beta \; \gamma_{1}r_{1}^{2}} & \gamma_{1} \\{\alpha \; \gamma_{2}\; r_{2}{\sin \left( \theta_{2} \right)}} & {{- \alpha}\; \gamma_{2}r_{2}{\cos \left( \theta_{2} \right)}} & {{\beta\gamma}_{2}\; r_{2}^{2}} & \gamma_{3} \\{{\alpha\gamma}_{3}\; r_{3}{\sin \left( \theta_{3} \right)}} & {{- {\alpha\gamma}_{3}}\; r_{3}{\cos \left( \theta_{3} \right)}} & {{\beta\gamma}_{3}\; r_{3}^{2}} & \gamma_{3} \\{{\alpha\gamma}_{4}\; r_{4}{\sin \left( \theta_{4} \right)}} & {{- {\alpha\gamma}_{4}}\; r_{4}{\cos \left( \theta_{4} \right)}} & {{\beta\gamma}_{4}\; r_{4}^{2}} & \gamma_{4}\end{bmatrix}\begin{bmatrix}{B_{x}(z)} \\{B_{y}(z)} \\{T(z)} \\{E(z)}\end{bmatrix}}} & (2)\end{matrix}$

where φ_(i)′(z) is the derivative of the measured phase change for corei as a function of distance down the fiber sensor 24.

Because the position of the fiber sensor is found by first measuring thephase change in each core and then calculated by integrating the bends,B_(x)(z) and B_(y)(z), along the fiber while accounting for the twist,τ(z), the inverse of this equation is needed:

$\begin{matrix}{\begin{bmatrix}{B_{x}(z)} \\{B_{y}(z)} \\{T(z)} \\{E(z)}\end{bmatrix} = {{\begin{bmatrix}{\alpha \; {\gamma \;}_{1}r_{1}{\sin \left( \theta_{1} \right)}} & {{- \alpha}\; \gamma_{1}r_{1}{\cos \left( \theta_{1} \right)}} & {\beta \; \gamma_{1}r_{1}^{2}} & \gamma_{1} \\{\alpha \; \gamma_{2}\; r_{2}{\sin \left( \theta_{2} \right)}} & {{- \alpha}\; \gamma_{2}r_{2}{\cos \left( \theta_{2} \right)}} & {{\beta\gamma}_{2}\; r_{2}^{2}} & \gamma_{3} \\{{\alpha\gamma}_{3}\; r_{3}{\sin \left( \theta_{3} \right)}} & {{- {\alpha\gamma}_{3}}\; r_{3}{\cos \left( \theta_{3} \right)}} & {{\beta\gamma}_{3}\; r_{3}^{2}} & \gamma_{3} \\{{\alpha\gamma}_{4}\; r_{4}{\sin \left( \theta_{4} \right)}} & {{- {\alpha\gamma}_{4}}\; r_{4}{\cos \left( \theta_{4} \right)}} & {{\beta\gamma}_{4}\; r_{4}^{2}} & \gamma_{4}\end{bmatrix}^{- 1}\begin{bmatrix}{\phi_{1}^{\prime}(z)} \\{\phi_{2}^{\prime}(z)} \\{\phi_{3}^{\prime}(z)} \\{\phi_{4}^{\prime}(z)}\end{bmatrix}} = {\overset{\_}{\overset{\_}{S}}\begin{bmatrix}{\phi_{1}^{\prime}(z)} \\{\phi_{2}^{\prime}(z)} \\{\phi_{3}^{\prime}(z)} \\{\phi_{4}^{\prime}(z)}\end{bmatrix}}}} & (3)\end{matrix}$

Here, S is known as the shape matrix.

The addition of a fiber amplifier in the measurement branch provides thebenefit of increased power to the sensor or DUT, but it also introducesan error in the OFDR measurement in the form of power fluctuationsbetween the rising and falling sweeps of the laser. An example of thesefluctuations are shown in the graph in FIG. 5, which illustrates thedifference in power levels between rising and falling laser sweeps thatare due to the differences in amplification between the sweeps performedover 10 nm at 2,937,500 GHz/s. The power level, detected using an HCNGas Cell connected in place of the sensor or DUT, (this is not shown inFIG. 4), shows a difference of more than 2 dB between rising and fallinglaser sweep measurements. Note that in FIG. 5 the rising laser sweepdata (bold black line) has been reversed for comparison with fallinglaser sweep data (thin black line); both are displayed from higher tolower optical frequencies.

Changes in the pump laser power result in a gain change of the EDFA witha response time that is a function of the fluorescence lifetime and thesignal and pump power in the erbium-doped fiber. When the pump laserpower is adjusted, there is a delay before the power at the output ofthe EDFA changes. This delay along with the gain response of the EDFA asa function of wavelength may affect the EDFA output as the tunable laseris swept. To compensate for how the EDFA's gain varies as a function ofwavelength a gain flattening filter (GFF) at the output filter may beadded. But there are gain differences even at the same wavelength thatvary based on sweep direction. Also, GFFs suffer fromtemperature-dependent wavelength shifts.

In a continuously swept OFDR system, OFDR measurement data is acquiredas the optical frequency of a tunable laser is increased (rising sweepof the laser) and as the optical frequency of the swept laser isdecreased (falling sweep of the laser). The light is amplified using anoptical amplifier, the gain of which is higher on falling sweeps than itis on rising sweeps. This gain difference can lead to less optimalsystem performance and less accurate measurement results.

To correct for unbalanced power during a sweep and from sweep to sweep,example embodiments vary the pump laser power between predeterminedpoints to maintain a substantially constant level of output power to thesensor fiber or device under test (DUT) in an OFDR system. In oneexample embodiment, the amplifier gain is modulated between two states:one for rising sweeps and one for falling sweeps. This embodiment allowsthe power to be balanced in the two laser sweep directions. Further, theamplifier gain may be modulated within a laser sweep with the additionof gain set points to flatten the power across the optical frequencyrange in which OFDR measurement data is acquired. Example embodimentsalso use turnarounds to make adjustments to balance the laser powerbetween rising and falling laser sweeps. One example adjustment includesadjusting the current driving the tunable laser's diode over the entiresweep cycle including rising, falling, and turnaround portions.

In example embodiments, to compensate for the sweep direction dependentgain difference, the data, acquisition, and control electronics 4adjusts the amplifier's pump laser power to pre-calibrated levels forthe rising and falling sweeps. Specifically, the system is configured toinclude two power states for the pump laser: one for the falling sweepand one for the rising sweep. (The adjustment may alternatively becontrolled by processor 5). An example of this pump laser powerswitching is illustrated in FIGS. 6A and 6B.

Looking at FIGS. 6A and 6B, the falling sweep power is left as is, andthe rising sweep power is increased to match that of the falling sweep.Because there is a response delay of the amplifier's gain, the pumplaser is driven to a higher power level following the completion of afalling sweep at the start of the laser turnaround. Setting the pumplaser power level at the start of the turnaround provides sufficienttime for the amplifier gain to be adjusted to the new level.

FIG. 7 compares the HCN power levels for rising (bold line) and falling(thin line) sweeps after the adjustment is applied to the amplifier'spump laser power, which reduces the average power difference between therising and falling laser sweeps to less than 0.02 dB.

In addition to adjusting pump laser power to compensate for sweepdirection dependent gain, the pump laser power can be adjusted toinclude more than 2 set points during the course of a laser's sweepcycle to further flatten the power to the DUT.

FIG. 8 is a flowchart diagram illustrating example procedures forimplementing an EDFA in an OFDR measurement system and for adjustingpump laser gain to balance the laser power output for rising and fallinglaser sweeps. Power requirements of the EFDA are determined (step S1),and the EDFA is configured to meet gain requirements including gainmedium type, length, and pumping directions (step S2). Placement of theEDFA in the optical network is determined, and the EDFA is added to theoptical network (step S3). The tunable laser is configured for a desiredsweep behavior (step S4). A gain difference is determined between risingand falling laser sweeps (step S5). Pump laser power set points areadded at specific optical frequencies to balance EDFA gain betweenrising and falling laser sweeps (step S6). The pump laser power setpoints are adjusted in order to balances the EDFA output power (stepS7).

Modulating Laser Output with a Known Signal

In a fiber optic sensing system, measurement of delay is important. Onedelay measured is the delay change between the cores of the multi-corefiber sensor. This delay change can be measured in terms of the phaseshift or phase difference from a reference phase for each core. In anOFDR system, there are many delay paths that an optical signal and thecorresponding electrical signal experience before being detected by theOFDR detection and acquisition system. One example delay, among manyothers, is that associated with the reference path of the measurementinterferometer to the photodiode through the detection and acquisitionsystem.

Example embodiments monitor changes in the delay of these paths andprovide a measurement that can be used to signal erroneous data and tocorrect for dynamic phase shifts in the reference path of the opticalnetwork. More specifically, by injecting a known signal, e.g., a ripplesignal with a known frequency, into the laser, the phase of each delaypath at that frequency can be computed. In one example implementation,that computation is done without performing a fast Fourier transform(FFT). Performing this measurement over a set of frequencies/wavelengthsproduces the phase response of each detection channel in the system.

In addition to adjusting the pump laser of an amplifier during the laserturnarounds, example embodiments perform in-system testing during laserturnarounds. In these example embodiments, the tunable laser diodeoutput in an optical (e.g., OFDR) measurement system is modulated todetect changes in optical and electrical delays within the system. Themodulation in the examples below includes injecting a known signal,e.g., a ripple with a known frequency, into the tunable laser diodeoutput. Other known signals or modulation techniques may be used.

Because constant laser power is desirable in an OFDR system, controlsystems and processing algorithms may be designed specifically to reduceor compensate for any ripple present in the tunable laser output. Atypical diode driver circuit that drives and maintains constant poweroutput is illustrated in FIG. 9. The diode driver 30 driving the laserdiode 32 may be a current source controlled via an analog and/or digitalclosed loop control scheme. The driver 30 may also be a look up table ofvalues stored in memory that are retrieved by a processor to adjust thecurrent to predetermined levels at specific locations in the laser'ssweep cycle in an open loop fashion. This table could also be updatedperiodically by an internal and/or external controlling process tomaintain laser power levels over time.

Unwanted modulations in the laser power can introduce errors in the OFDRmeasurement in the form of broad band noise or as reflective events thatare not actually present in the sensor or DUT. Contrary to conventionalthinking, example embodiments purposely inject ripple into the laserdiode output signal with steps taken so as not to degrade the OFDRmeasurement.

FIG. 10 depicts the laser diode driver 30 along with a modulator 34whose outputs are combined by combiner 36 and provided to the laserdiode 32. The modulator 34 can be implemented in various ways andcombined with the diode driver 30 signal in various ways, such as forexample wiring the signals together or adding them with an op-ampconfigured as an adder.

One example of modulator 34 is shown below in FIG. 11 and includes aprocessor-controlled digital-to-analog converter (DAC) 40,voltage-controlled oscillator (VCO) 42, and an active filter 44providing signal gain control and low pass filtering. In this exampleembodiment, the processor 5 selects the VCO 42 output signal frequencyby setting the DAC 40 output voltage to a desired value. The VCO 42output signal may then be used as the modulator by adding this signal tothe laser diode drive signal. Additional filtering/amplification 44 maybe applied depending on the desired signal characteristics such asamplitude and noise levels. Example processors 5 include a fieldprogrammable gate array (FPGA), a microcontroller, a digital signalprocessor, or other processors. One disadvantage of this modulationapproach is that to measure the phase difference between the modulatedsignal and detected optical signal, an extra data acquisition channel isrequired to measure the frequency of the VCO output.

Another example embodiment of modulator 34 is shown in FIG. 12 andincludes processor 5 having or controlling a numerically-controlledoscillator (NCO) 46. The NCO 46 produces a digital output thatrepresents a sine wave at a desired frequency. From this digital output,a clock signal is generated, corresponding to the most significant bit(MSB) 48 of the NCO output. The digital clock signal, output from theprocessor 5, is low pass filtered by filter/shaper 50 to band limit thecontent of the digital clock signal. One example filter/shaper 50 is anRC Filter. In this example embodiment, the filtered clock signal is themodulation signal added to the laser diode drive signal to inject rippleto the laser output. This technology requires only minimal externalcomponents and has the advantage that the ripple signal (clock) issourced directly from the processor 5. As a result, the processor 5knows the frequency of the ripple signal so there is no extrameasurement required to determine the frequency of the modulated signal.These features are advantageous when performing high resolution phasedifference measurements in the OFDR system.

Driving the laser diode with the addition of this type of modulatorproduces laser light that carries a known frequency ripple signalcomponent through all paths of the optics and electronic detectioncircuitry, e.g., a known amplitude modulation. Measuring the light thathas traversed the optical network's reference paths, the amplitudemodulated signal at each measurement channel in the OFDR system isdetected in the data acquisition electronics 4. The phase difference ofripple signal detected on each channel (each channel corresponds to acore in the multi-core fiber sensor) from the originally injected rippleat the modulated frequency is measured, and this phase difference is ameasurement of optical and electrical delay differences betweenchannels.

An example modulation and measurement system is shown in FIG. 13. As anexample, suppose the NCO 46 is set to 12.5 MHz with the MSB 48 beingused as the clock signal sent to filter/shaper 50 and combined at 36with the laser output from the diode driver 30. The modulated driveroutput then drives the tunable laser 32 used to sweep the opticalnetwork 2. The optical network response for N channels is detected byphotodiodes 52, and the corresponding N electrical signals areconditioned 54 and converted to N digital signals corresponding to Ncore ODFR measurement channels by ADC 56. The N channel digital signalsare processed by the DSP or processor 58 to produce N different channelphases and to calculate and output N−1 phase difference signals betweeneach determined channel phase with respect to one common channel'sphase. The common channel may be selected as one of the OFDR channels.

Data from the N channels is acquired by the processor 5 in the timedomain, meaning that each point is sampled at particular frequency,e.g., at 200 MHz. To determine the phase of the injected ripple for eachchannel, a fast Fourier transform (FFT) is performed on the measurementdata from each channel without having to perform data linearization.This is unlike processing for a sensor/DUT OFDR measurement where theOFDR measurement data must first be linearized or resampled based on thelaser tuning speed during the course of the measurement. Once the FFT isperformed on the non-linearized measurement data, a peak is observed inthe data at index 12,500 corresponding to a 12.5 MHz modulation signal(in this example). To determine the phase difference between the Nchannels, the DSP 58 takes the FFT result and computes the phase of thecomplex point at index 12,500 for each channel to determine the phase ofthe signal detected at the modulation frequency. Then one common channel(example channel 0) is subtracted from all N channels. These resultantN−1 phase differences from each of N−1 channels with respect to commonchannel 0 indicates N−1 delay differences between the N cores in thefiber sensor. The N−1 delay differences between the N cores in the fibercan produce a significant error, e.g., in a shape sensing application,if not removed from the OFDR measurement.

By sweeping the NCO 46 over the frequency range of the DUT or sensor,the phase of each OFDR measurement channel at each frequency is computedyielding the phase response of the optical network's reference path andthe electronic acquisition and detection circuitry for each coremeasurement channel as a function of frequency. This phase response isused to correct for the phase differences between each measurementchannel. Additionally, monitoring this phase response for changesprovides real-time monitoring of the delay paths in an optical networkand in the electronics, which may be used to provide feedback to detectmeasurement errors induced as a result of optical and electrical delaypath changes.

The inventors realized that because the phase of only a single point ofthe FFT is required, the phase can be computed without performing acomplete FFT by mixing the data acquired from the N channels to basebandusing the same frequency as the modulator, which in this example is theoutput of the NCO used to source the modulation signal. Once atbaseband, the DC term of each channel is the point at which the phase isto be computed and compared to the common channel. The phase is computedby accumulating complex components of the basebanded signal of eachchannel and computing the phase of the resulting complex value. Theseoperations can be performed for example in a DSP or FPGA.

An example of such DSP processing is shown in FIG. 14. The N digitalsamples from the N core OFDR measurements are mixed in mixer 60 with theoutput signal from the NCO 46. The mixer 60 multiples each channel bythe complex NCO 46 output (sine and cosine). The complex mixed signalsare accumulated (summed) at 62 and converted from complex to polar formto determine the phase. These phase values represent the phasedifference between the modulated signal and the signals detected foreach channel. To obtain the phase difference between the channels, thephase of one common channel is subtracted at 66 from each of the Nchannels to determine the N−1 phase differences between the channels.

FIG. 15 is a flowchart illustrating example procedures for performingdelay monitoring using laser diode ripple injection. Electronics areadded to the OFDR system to modulate the laser diode at one or morefrequencies (step S10), and the frequency(frequencies) of the injectedripple is(are) selected (step S11). The added ripple is located in afrequency or frequencies that are out of the sensor's measurementfrequency range to ensure that the added ripple does not interfere withsensor measurements (step S12). The ripple signal is injected into thelaser diode output (step S13), and OFDR data and phase measurements aremade for all monitored channels at the modulation frequency(frequencies)(step S14). The DSP computes the phase difference of the modulationsignal between N channels to determine if a change in optical orelectrical delay occurred (step S15).

Using Laser Sweep Edges and Turnarounds to Perform AdditionalMeasurements

While the modulated laser light may interfere with the sensormeasurement, it is also possible that the interference is caused byreflected events in the fiber sensor. The inventors realized that theeffects of linearization on this modulation signal needed to beunderstood and accounted for. Linearization refers to the process ofresampling the acquired measurement data based on the instantaneoustuning rate of the laser. The result of the linearization process ismeasurement data which is equally-spaced in optical frequency ratherthan in time.

FIG. 16A shows how an out-of-band modulation without linearizationproduces a strong observable peak relative to sensor gratings at thefrequency of the modulation source. Linearization is not required tomeasure the modulation signal because this is just the measurement of anamplitude modulated signal in the time domain measurement and not anOFDR measurement. FIG. 16B illustrates the effect of linearization onsuch a signal. The resampling that is done as part of the linearizationprocess spreads this signal over a wide range of frequencies based onthe tuning rate fluctuations of the swept laser.

Modulating the laser power can interfere with the measurement beingperformed if the modulation frequency and timing of this signal are notconsidered. For instance if the modulation signal is too close to sensorgratings included along the fiber sensor, it is possible that when thedata is linearized, the modulation signal may interfere with the OFDRgrating measurement. In an OFDR-based measurement, modulation of thelaser diode can be done at the same time as the OFDR measurementprovided that the modulation frequency is outside the frequenciesexpected of the OFDR measurement. However, care must be taken to ensurethat fiber sensor connector and termination reflections are minimized.Otherwise, these reflections can create interference signals whencoupled with the modulation signal that produce additional unwantedfrequencies of modulation within the fiber sensor that can introduceerrors in the OFDR measurement.

If a grating measurement is being performed, a higher grating strengthcan corrupt the measurement of the modulation signal. The inventorsrecognized several options to address this problem. One option is tolimit the modulation frequencies to frequencies outside that of thegrating fiber. A disadvantage of this option is possible measurementerror since the frequencies where the sensor actually resides are notbeing measured. This disadvantage may be removed by moving themodulation from frequencies within the measurement region to theturnaround frequencies. Depending on the response of the amplifier andduration of the turnarounds, the amplifier can be turned off entirely orthe amplifier gain reduced. Reducing the amplifier gain sufficiently canreduce the grating reflections to below the noise floor, therebyallowing the modulation measurement to be performed without anymeasureable interference from the gratings.

To avoid measurement interference, example embodiments perform laserdiode modulation based measurements during turnaround points of thelaser sweep as illustrated in FIGS. 17A and 17B. The example of FIG. 17Bshows the modulation occurring only during the turnaround times,although there may be embodiments where some modulation may occuroutside the turnaround times or continuously.

The inventors realized that these laser sweep turnaround points providean opportunity to make further corrections, perform checks, and makeother dynamic adjustments. This continuous sweeping action of the lasercan be further broken down into the different sections illustrated inthe graph in FIG. 18.

One example embodiment uses laser sweep turnarounds to performadditional OFDR measurement data acquisitions that are beyond the fibersensor wavelength. For example, in the case of a fiber sensor thatincludes Bragg gratings, grating reflections can be designed or thefiber restricted to reflect light within a known wavelength range. Thecontinuous laser sweep is configured so that the turnarounds are beyondthe wavelength range in which those OFDR measurements are acquired fromthe fiber sensor, i.e., beyond the fast rising and fast falling portionsof the laser sweep shown in FIG. 19. This allows OFDR measurements to bemade over additional wavelength ranges in the turnaround parts of thelaser sweep. Extending the turnarounds to include additional ranges alsoallows for Rayleigh scatter-based OFDR measurements to be performed(which are different form the Bragg grating-based OFDR measurements).

In addition to acquiring data beyond the frequency range of thegratings, or the primary OFDR measurement, out of band OFDR measurementscan be taken at slower sweep rate portions of the laser sweep asillustrated in FIG. 19. By slowing the laser sweep rate down in theseportions, out-of-band OFDR measurements can be performed over a longerdistance along the fiber sensor.

Example extension of the OFDR measurement range is illustrated in FIGS.20A and 20B. FIG. 20A depicts an example fiber Bragg grating amplitudemeasurement at a faster sweep rate and shorter delay or length along thefiber sensor. Utilizing a wider laser sweep or scan range provideshigher resolution, but the faster sweep rate results in a shortermeasurement range as the Nyquist rate (i.e., the minimum rate at which asignal can he sampled without introducing errors, which is twice thehighest frequency present in the signal) is located at a shorterdelay/distance from 0 ns. FIG. 20B shows a graph of a slower, lowerresolution sweep. As a result of the slower sweep rate and utilizingoptical frequencies that are beyond the range of the gratings,reflective events beyond those of the sensing fiber can be measured.

FIG. 21 is a flowchart illustrating example procedures for utilizinglaser sweep edges and turnarounds to perform additional measurements.The laser is configured to perform a desired sweeping behavior includingfor example desired sweep rate and power for making additionalmeasurements (step S20). Wavelength specific data acquisition startlocation and acquisition specific parameters such as the wavelengthrange of the measurement and the desired number of points in themeasurement are determined (step S21). The data acquisition circuitryacquires and processes ODFR measurements over the wavelength rangespecified (step S22).

FIG. 22 illustrates an example of an OFDR system that includes multiplefeatures from above such as a laser modulator such as one of theexamples described above and frequency-specific power levels to adjustthe pump laser's power level. In accordance with example embodiments, atable of frequency-specific power levels stored in memory 35 is used byprocessing control circuitry in the detection, acquisition, and controlelectronics 4 to control the pump laser power level. The modulator 34output combines with the output of the diode driver circuit 30 tocontrol the output power of the tunable laser 1, e.g., for the reasonsdescribed in an earlier section, and/or to inject the modulated signalinto the laser output, e.g., for the reasons just described in thissection.

The technology described above has wide and diverse applications toimprove the accuracy and reliability of optical network measurements.One non-limiting example application for shape sensing fiber coupled toan OFDR measurement instrument that requires a high degree of confidencein terms of the accuracy and reliability of the shape sensing output isrobotic arms used in surgical or other environments. FIG. 23 shows anexample use of a fiber optic shape sensing system for a robotic surgicalarm in which one or more of the various technical features and/orembodiments described above may be used.

Although various embodiments have been shown and described in detail,the claims are not limited to any particular embodiment or example. Noneof the above description should be read as implying that any particularelement, step, range, or function is essential such that it must beincluded in the claims scope. The scope of patented subject matter isdefined only by the claims. The extent of legal protection is defined bythe words recited in the allowed claims and their equivalents. Allstructural and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the technology described, for it to beencompassed by the present claims. No claim is intended to invokeparagraph 6 of 35 USC § 112 unless the words “means for” or “step for”are used. Furthermore, no embodiment, feature, component, or step inthis specification is intended to be dedicated to the public regardlessof whether the embodiment, feature, component, or step is recited in theclaims.

1-7. (canceled)
 8. An Optical Frequency Domain Reflectometry (OFDR) interrogation system for measuring an optical fiber sensor including multiple optical cores, the OFDR interrogation system comprising: a tunable laser configured to sweep laser light over; a modulator configured to add a known modulation to an output of the tunable laser to produce swept laser light, the swept laser light including the known modulation; an optical interferometric network configured to provide the swept laser light to the optical fiber sensor and to output reflected light from the optical fiber sensor, wherein the outputted reflected light includes, for each core of the multiple optical cores, a reflection of the swept laser light, and wherein the outputted reflected light corresponds to sensor data; and data processing circuitry configured to: determine, based on the known modulation, a phase difference between the outputted reflected light associated with different optical cores of the multiple optical cores, and process the sensor data based on the phase difference.
 10. The OFDR interrogation system of claim 8, wherein the data processing circuitry is further configured to monitor a phase response of the outputted reflected light from the optical fiber sensor to detect measurement errors from delay path changes.
 10. The OFDR interrogation system of claim 8, further comprising a laser driver with an output, wherein the laser driver drives the tunable laser to sweep the laser light using the output, and wherein the modulator is coupled to the output of the laser driver.
 11. The OFDR interrogation system of claim 8, wherein the modulator includes a controller coupled to a digital to analog converter and a filter, wherein the digital to analog converter is configured to drive a voltage-controlled oscillator, and wherein the filter is configured to filter an output from the voltage-controlled oscillator to generate a signal used to add the known modulation.
 12. The OFDR interrogation system of claim 8, wherein the modulator includes a numerically-controlled oscillator and a filter, wherein the numerically-controlled oscillator is configured to generate a binary signal having a most significant bit used to provide a clock signal, and wherein the filter is configured to filter the clock signal to generate a signal used to add the known modulation. 13-14. (canceled)
 15. The OFDR interrogation system of claim 8, wherein the OFDR interrogation system is configured to measure the optical fiber sensor in a first measurement range of wavelengths, wherein the tunable laser is configured to sweep the laser light by including: a rising sweep where a laser light frequency increases from a lower optical frequency of the first measurement range to a higher optical frequency of the first measurement range, a falling sweep where the laser light frequency decreases from the higher optical frequency to the lower optical frequency, and a turnaround portion transitioning between the rising and falling sweeps, and wherein the modulator is configured to add the known modulation to the swept laser light during the turnaround portion.
 16. The OFDR interrogation system of claim 8, wherein the OFDR interrogation system is configured to measure the optical fiber sensor in a first measurement range of wavelengths, and wherein the modulator is configured to add the known modulation by: adding a modulation with a wavelength outside the first measurement range of wavelengths. 17-28. (canceled)
 29. A method of using an optical frequency domain reflectometry (OFDR) interrogation system to measure an optical fiber sensor including multiple optical cores, the method comprising: adding a known modulation to an output of a tunable laser to produce swept laser light including the known modulation; using an optical interferometric network to provide the swept laser light to the optical fiber sensor and to output reflected light from the optical fiber sensor, wherein the outputted reflected light includes, for each core of the multiple optical cores, a reflection of the swept laser light, and wherein the outputted reflected light corresponds to sensor data; determining, based on the known modulation, a phase difference between the outputted reflected light associated with different optical cores of the multiple optical cores, and processing the sensor data based on the phase difference.
 30. (canceled)
 31. The method of claim 29, wherein the OFDR interrogation system is configured to measure the optical fiber sensor in a first measurement range of wavelengths; wherein sweeping laser light over the first measurement range of wavelengths comprises: including a rising sweep where a laser light frequency increases from a lower optical frequency of the first measurement range to a higher optical frequency of the first measurement range, including a falling sweep where the laser light frequency decreases from the higher optical frequency to the lower optical frequency, and including a turnaround portion transitioning between the rising and falling sweeps; and wherein adding the known modulation comprises: adding the known modulation during the turnaround portion.
 32. The method of claim 29, wherein the OFDR interrogation system is configured to measure the optical fiber sensor in a first measurement range of wavelengths, and wherein adding the known modulation comprises: adding a modulation with a wavelength outside of the first measurement range of wavelengths. 33-39. (canceled)
 40. The OFDR interrogation system of claim 8, wherein the data processing circuitry is configured to process the sensor data based on the phase difference by: determining a delay difference between the different optical cores based on the phase difference; and correcting for the delay difference.
 41. The OFDR interrogation system of claim 8, further comprising: detection circuitry configured to convert the outputted reflected light from the optical fiber sensor into corresponding signals; and wherein the data processing circuitry is configured to determine the phase difference by: mixing a signal used to create the known modulation with the corresponding signals with to produce mixed signals, accumulating the mixed signals to produce accumulated signals, and determining phases of the corresponding signals using the accumulated signals.
 42. The OFDR interrogation system of claim 8, wherein the known modulation comprises an amplitude modulation.
 43. The OFDR interrogation system of claim 8, wherein the data processing circuitry is further configured to: determine a shape of the optical fiber sensor based on the processed sensor data.
 44. The method of claim 29, wherein processing the sensor data based on the phase difference comprises: determining a delay difference between the different optical cores based on the phase difference; and correcting for the delay difference.
 45. The method of claim 29, wherein the known modulation comprises an amplitude modulation, the method further comprising: determining a shape of the optical fiber sensor using the processed sensor data.
 46. A non-transitory machine-readable medium comprising a plurality of machine-readable instructions which when executed by one or more processors associated with an optical frequency domain reflectometry (OFDR) interrogation system, the OFDR interrogation system configured to measure an optical fiber sensor including multiple optical cores, are adapted to cause the one or more processors to perform a method comprising: adding a known modulation to an output of a tunable laser to produce swept laser light including the known modulation; using an optical interferometric network to provide the swept laser light to the optical fiber sensor and to output reflected light from the optical fiber sensor, wherein the outputted reflected light includes, for each core of the multiple optical cores, a reflection of the swept laser light, and wherein the outputted reflected light corresponds to sensor data; determining, based on the known modulation, a phase difference between the outputted reflected light associated with different optical cores of the multiple optical cores, and processing the sensor data based on the phase difference.
 47. The non-transitory machine-readable medium of claim 46, wherein the OFDR interrogation system is configured to measure the optical fiber sensor in a first measurement range of wavelengths; wherein sweeping laser light over the first measurement range of wavelengths comprises: including a rising sweep where a laser light frequency increases from a lower optical frequency of the first measurement range to a higher optical frequency of the first measurement range, including a falling sweep where the laser light frequency decreases from the higher optical frequency to the lower optical frequency, and including a turnaround portion transitioning between the rising and falling sweeps; and wherein adding the known modulation comprises: adding the known modulation during the turnaround portion.
 48. The non-transitory machine-readable medium of claim 46, wherein the OFDR interrogation system is configured to measure the optical fiber sensor in a first measurement range of wavelengths, and wherein adding the known modulation comprises: adding a modulation with a wavelength outside of the first measurement range of wavelengths.
 49. The non-transitory machine-readable medium of claim 46, wherein processing the sensor data based on the phase difference comprises: determining a delay difference between the different optical cores based on the phase difference; and correcting for the delay difference. 